With modern internal combustion engines the required combustion air is often supplied to the cylinders via a supercharger with a pressure increased in relation to the ambient pressure. The pressure increase required for this can in this case typically be achieved by using a so-called mechanical supercharger. This is driven via the crankshaft of the internal combustion engine.
Also known is the practice of increasing the pressure via a so-called exhaust gas turbocharger. In this case a turbo compressor is driven with the aid of a turbine connected to the exhaust system which ensures an increase in pressure on the air inlet side of the cylinders. For this type of turbocharging it is however also known that at low engine speeds and the associated low mass throughputs a so-called turbo lag occurs. The efficiency of the turbocharger is then low in this operating range and only a relatively small motor torque can be achieved at full load.
Such a turbocharger and its use in an internal combustion engine are for example known from DE 10 2004 030 B3. This document is based on control of the internal combustion engine with a turbocharger in which, depending on a requested torque, the inlet and exhaust valves are adjusted so that these are opened simultaneously for a predetermined time, meaning that their timing overlaps. With a four-stroke engine, this valve overlapping occurs towards the end of the 4th stroke on expulsion of the exhaust gas and continues into the 1st stroke of the induction, so that the inlet valve is already open before the exhaust valve is completely closed. This means that the accelerated exhaust gas column in the exhaust manifold causes fresh fuel-air mixture to be sucked into the combustion chamber and reduces the likelihood of exhaust gas remaining in the combustion chamber.
With high-performance engines the exhaust valve opens even before bottom dead center is reached, this relaxes the gas column early, so that the subsequent upwards movement of the piston is not so heavily braked. During this upwards movement the inlet valve then also opens, so that, as mentioned, both are now simultaneously opened. Since the gas mixture has however been sharply accelerated in the direction of the exhaust valve no air is pushed back at the inlet valve, but a flushing process takes place which is also promoted by a large valve acceleration which additionally carries air with it. The exhaust valve ideally closes when the air column comes to a standstill. The inlet valve likewise remains open until the air column which is sucked in by the upwards movement of the piston comes to a standstill. Since the air is sluggish to a certain extent at high engine speeds, it now compresses itself against the piston which is already moving upwards again. This additional compression brings about a better filling.
With this process, which is also referred to as scavenging, a fall in pressure occurs from the inlet side to the outlet side across the cylinders so that a flushing through of inlet air to the exhaust side occurs. The result is that any turbo lag occurring is reduced. To increase the accuracy with which the mass of air actually remaining in the cylinder can be determined during or after a scavenging process, it is additionally proposed, if the actual Lambda value deviates from Lambda setpoint value, for a valve overlap to correct the value for the opposing exhaust pressure as a function of the deviation established.